Our copending British Application GB No. 2172118A discloses a system for producing colour proofs from electronically stored image data. In particular there is disclosed a radiation-sensitive element suitable for colour proofing comprising a substrate bearing at least four separate imaging media coated thereon, said imaging media including:
(1) an imaging medium capable of forming a yellow image upon imagewise exposure and processing, PA1 (2) an imaging medium capable of forming a magenta image upon imagewise exposure and processing, PA1 (3) an imaging medium capable of forming a cyan image upon imagewise exposure and processing, and PA1 (4) an imaging medium capable of forming a black or a balancing black image upon imagewise exposure and processing, PA1 (1) an imaging medium capable of forming a yellow image upon imagewise exposure and processing, PA1 (2) an imaging medium capable of forming a magenta image upon imagewise exposure and processing, and PA1 (3) an imaging medium capable of forming a cyan image upon imagewise exposure and processing, PA1 The system employing `dye developers` which are dye molecules linked to one or more hydroquinone developing moiety. This is described in detail in "The Chemistry of Synthetic Dyes", K. Venkataraman, Vol. VIII, Chapter 8, New York, Academic Press, 1978. PA1 The system employing "Redox dye release" molecules such as those of the ortho- or parasulphonamidophenol or sulphonamidonaphthol types described, for example, in "The Theory of the Photographic Process", T. H. James, 4th Edition, page 370 New York, Macmillan, 1977. PA1 The system employing sulphonomethylene derivatives of quinones as described in European Patent Application No. 4,399.
each imaging medium having a maximum spectral sensitivity at a wavelength different from that of the maximum sensitivity of the other imaging media.
There is also disclosed a process for producing a coloured half-tone image comprising providing a light sensitive element comprising a substrate bearing three separate imaging media coated thereon, said imaging media consisting of:
each imaging medium having a maximum spectral sensitivity at a wavelength different from that of the maximum spectral sensitivity of the other imaging media and a sensitivity at the wavelength of maximum spectral sensitivity of any of said other imaging media which is not significant compared to the maximum sensitivity of said other media, exposing said element to three independently modulated sources each emitting radiation of a wavelength corresponding to the waveength of maximum sensitivity of a respective imaging medium, said exposure being conducted in raster fashion.
In order to ensure that only the intended layer is exposed by any particular light source it is very desirable that the sensitive layers should exhibit high contrast or, more precisely, a very short exposure range from maximum density (Dmax) to minimum density (Dmin). A high photographic contrast is also a requirement for accurate recording of halftone images where it is desirable that exposure will generate either a full response or zero response. Preferably the sensitometric contrast of each imaging layer is sufficiently high that the difference between the exposure required to give a density which is 5% of the maximum density above fog and the exposure required to give a density which is 90% of the maximum density above fog is less than 1.5 log exposure units.
For the purposes of colour proof recording, there are several important advantages in producing the image in halftone form which are fully discussed in British Patent Application GB No. 2172118A. In particular halftone reproduction via electronically generated halftone dots gives a greater tolerance to variation in output of the exposing devices as well as to variations in the processing chemistry for the recording materials than would be the case for continuous tone recording. Image recording in halftone form also places much less stringent requirements on balancing the density and exposure characteristics and processing rates of the individual image forming layers.
These and other advantages follow from the nature of halftone recording which requires only maximum and minimum densities to be formed in the image layers. There is also a subjective advantage to proofing in halftone form since this is the structure of the final printed image as well as of the highest quality proofs which can be produced with present methods.
Although there are very good reasons for generating actual colour proofs in halftone format, images in this form are not suitable for re-scanning on a colour separation scanner should this be necessary.
There is a significant demand for colour hardcopy from electronically stored image data, output in a form which can be re-scanned. This arises from the demand for "second generation originals"; that is, original photographs which have been electronically retouched or otherwise modified and then copied onto fresh colour film to yield a clean `original` which may be sent to other locations and subsequently scanned on a colour separation scanner. The Dr. Rudolf Hell Gmbh Colour Proof Recorder CPR403 which outputs continuous tone, colour hardcopy on standard colour paper or transparency is presently in use for the production of such second generation originals.
It has now been found that a colour proof recording system such as that disclosed in British Patent Application GB No. 2172118A can be modified to include the possibility of producing continuous tone second generation colour originals.
In this case the image produced should match, as closely as possible, the original which may be a colour transparency on Kodak Ektachrome film or corresponding materials. Only a three colour material is therefore required (yellow, magenta and cyan). To produce continuous tone images the LED arrays must be controlled so that they may be driven at any power levels between the limits giving Dmax and Dmin. Therefore in order to maximise reproducibility of tonal values, the exposure range of the individual sensitive layers of the material should match the practical intensity range of the LED sources.